A MEMS microphone is also called a microphone chip or silicon microphone. MEMS microphones are usually referred to as being of two main types: analog and digital. Both types are based on a membrane or diaphragm that is combined with a permanently charged capacitor that changes its capacitance according to the pressure derived from acoustic waves. This is commonly known as an electret microphone. The pressure-sensitive diaphragm is etched directly into a silicon chip by MEMS techniques, and is usually accompanied with a preamplifier; this is referred to as an ‘analog MEMS microphone’. To be more readily integrated with modern digital products an external analog-to-digital converter (ADC) is usually used together with the analog MEMS microphone. “Digital MEMS microphones” include an ADC circuit in the same package.
An external power supply is required by both the analog and digital MEMS microphone. In the case of an analog MEMS microphone, the power is consumed by the integrated preamplifier. A typical electret microphone preamplifier circuit uses an Field Effect Transistor (FET) in a common_source configuration which must be externally powered by a supply voltage.
It is becoming more common, however, for the preamplifier to be a low power operational amplifier. The analog MEMS microphone represents the lowest power consumption case. Digital MEMS microphones consume more power as they also contain an integrated ADC in addition to the amplifier. The use case is that the microphone in a mobile or wearable device needs to be active, even when the device is sleeping, such that it can detect any wake up voice commands. Hence it is highly desirable that the MEMS microphone consumes ultra-low power. Target power consumption is in the order of less than 25 microwatts whereas the usual power consumption of an analog MEMS microphone is in the order of 200 microwatts and digital MEMS microphones consume more.